1. Field of the Invention
The present invention relates generally to phase shift masks, and particularly to a structure of a phase shift mask of attenuation type attenuating light intensity, and a method of manufacturing the same. The present invention further relates to an exposure method using the phase shift mask.
2. Description of the Background Art
Recently, high integration and miniaturization of a semiconductor integrated circuit has been remarkably advanced, involving miniaturization of a circuit pattern formed on a semiconductor substrate (hereinafter referred to simply as a wafer).
In particular, a photolithography technique is well known as a basic technique in pattern formation. Although various developments and improvements have been made, miniaturization of a pattern still keeps on advancing, and a demand for enhancement of a pattern resolution has been increasing.
In general, a resolution limit R (nm) in a photolithography technique using a reduction exposure method is described by the following: EQU R=k.sub.1 .multidot..lambda./ (NA) (1)
where .lambda. is a wavelength (nm) of light to be used, NA is a numerical aperture of a lens, and k.sub.1 is a constant which depends on a resist process.
As can be seen from the above expression, in order to enhance the resolution limit, k.sub.1 and .lambda. should be made smaller, and NA should be made larger. That is, the wavelength should be decreased, and NA should be increased, with the constant which depends on a resist process made smaller.
However, improvement of a light source and a lens is technically difficult. In addition, a depth of focus .delta. (.delta.=k.sub.2 .multidot..lambda./(NA).sup.2) of light is made smaller by decreasing the wavelength and increasing NA, which rather leads to deterioration in the resolution.
Description will now be made of a cross section of a mask, an electric field of exposure light on the mask, and light intensity on a wafer in using a conventional photomask, with reference to FIGS. 31A, 31B, 31C.
First, the cross sectional structure of the mask will be described with reference to FIG. 31A. A metal mask pattern 2 of chromium or the like is formed on a glass substrate 1.
Referring to FIG. 31B, an electric field is generated along the mask pattern. Referring to FIG. 31C, however, light beams passing through the mask intensify each other at an overlaid portion of the light beams caused by light diffraction and interference. Consequently, the difference in the light intensity on the wafer becomes smaller, so that the resolution is deteriorated.
A phase shift exposure method with a phase shift mask has been proposed for solving this problem, for example, in Japanese Patent Laying-Open Nos. 57-62052 and 58-173744.
The phase shift exposure method with a phase shift mask disclosed in Japanese Patent Laying-Open No. 58-173744 will now be described with reference to FIGS. 32A, 32B, 32C.
FIG. 32A shows a cross section of the phase shift mask. FIG. 32B shows an electric field on the mask. FIG. 32C shows light intensity on a wafer.
First, referring to FIG. 32A, a phase shifter 6b of a transparent insulation film such as a silicon oxide film is provided at every other aperture portion 6a of a chromium mask pattern 2 formed on a glass substrate 1 to form a phase shift mask.
Referring to FIG. 32B, the electric field of a light beam passing through phase shifter 6b of the phase shift mask is inverted by 180.degree..
Therefore, the light beams transmitted through aperture portion 6a and through phase shifter 6b cancel each other on an overlaid portion thereof caused by a light interference effect. Consequently, as shown in FIG. 32C, the difference in the light intensity on the wafer is sufficient for enhancing the resolution.
Although the aforementioned phase shift mask is very effective for a periodical pattern such as lines and spaces, it cannot be set to an arbitrary pattern because complexity of the pattern causes great difficulty in arrangement of a phase shifter and the like.
As a phase shift mask solving the above problem, a phase shift mask of attenuation type is disclosed, for example, in JJAP Series 5 Proc. of 1991 Intern. Microprocess Conference pp. 3-9 and Japanese Patent Laying-Open No. 4-136854. Description will hereinafter be made of the phase shift mask of attenuation type disclosed in Japanese Patent Laying-Open No. 4-136854.
FIG. 33A is a cross section of the phase shift mask of attenuation type. FIG. 33B shows an electric field on the mask. FIG. 33C shows light intensity on a wafer.
Referring to FIG. 33A, the structure of a phase shift mask 100 includes a quartz substrate 1 transmitting exposure light, and a phase shift pattern 30 having a prescribed exposure pattern including a first light transmit portion 10 formed on a main surface of quartz substrate 1 and having the main surface exposed, and a second light transmit portion 20 converting a phase of transmitted exposure light by 180.degree. with respect to the phase of exposure light transmitted through first light transmit portion 10.
Second light transmit portion 20 has a double layer structure including a chromium layer 2 having the transmittance of 5-40% for exposure light, and a shifter layer 3 converting a phase of exposure light transmitted therethrough by 180.degree. with respect to that of exposure light transmitted through light transmit portion 10.
The electric field on the mask, of exposure light passing through phase shift mask 100 having the above-described structure is as shown in FIG. 33B. The light intensity on the wafer has its phase inverted at an edge of the exposure pattern as shown in FIG. 33C.
The light intensity at an edge of the exposure pattern, therefore, is invariably 0, as shown in the figure, so that the difference in the electric field on light transmit portion 10 and phase shifter portion 20 of the exposure pattern is sufficient for high resolution.
It should be noticed that the transmittance of second light transmit portion 20 for exposure light is set to 5-40% in the above method, for the purpose of adjusting the thickness of the resist film after development thereof by the transmittance, as shown in FIG. 30, so as to adapt the exposure amount appropriately for lithography.
Description will now be made of a method of manufacturing phase shift mask 100. FIGS. 35 to 39 are cross sectional views showing the manufacturing steps according to the cross section of phase shift mask 100 shown in FIG. 33.
Referring to FIG. 35, chromium film 2 having the exposure light transmittance of 5-40% and the thickness of 50-200 .ANG., approximately, is formed on glass substrate 1. Thereafter, on chromium film 2 formed is SiO.sub.2 film 3 of a prescribed thickness having the phase of exposure light passing therethrough converted by 180.degree.. An electron beam resist film 5 is formed on SiO.sub.2 film 3.
Referring to FIG. 36, a predetermined portion of electron beam resist film 5 is exposed to electron beams and developed to form a resist 5 having a desired pattern.
Referring to FIG. 37, with resist film 2 as a mask, the SiO.sub.2 film is etched using a gas of the CHF.sub.3 family. Referring to FIG. 38, chromium film 2 is subjected to wet etching with resist film 5 and SiO.sub.2 film 5 as a mask.
Referring to FIG. 39, phase shift mask 100 is completed by removing resist film 5.
In the above conventional technique, however, second light transmit portion 20 has a double layer structure including chromium film 2 for controlling the transmittance and SiO.sub.2 film 3 for controlling the phase difference. This structure, therefore, requires devices and process respectively for formation of a chromium film and an SiO.sub.2 film.
In addition, the chromium film and the SiO.sub.2 film must be etched separately with different etching agents, resulting in numerous steps of the process, and thus leading to higher probabilities of defects and of process errors in the pattern dimension.
Referring to FIG. 40, when a remaining defect (opaque defect) 50 and a pinhole defect (clear defect) 51 should occur in the phase shift mask pattern, repairing methods respectively applicable to a chromium film and an SiO.sub.2 film will be required for repairing the defect. A conventional repairing method, therefore, cannot be employed.
Referring to FIG. 41, according to an exposure method using the above-described phase shift mask 100, the film thickness of a second light transmit portion 20 of phase shift mask 100 is approximately 3050 .ANG. to 4200 .ANG., which is relatively large. Therefore, as shown in the figure, oblique exposure light out of exposure light from an exposure light source has its phase not reliably converted by 180.degree. even if it transmits through second light transmit portion 20 of phase shift mask 100. Exposure light having a different phase is produced.